1. Field or the Invention.
This invention relates to an improved method and apparatus for calibrating scanning infrared detector systems. More particularly, it relates to a simple, stable calibration method and apparatus which does not require close temperature control of the source and which produces repeatable AC calibration signals to the detection system.
2. Description of the Prior Art.
Most spatial scanning and infrared (IR) imaging systems involved in measurement or detection applications require some form of radiometric calibration. Calibration in this context refers to the determination of the relationship (i.e., responsivity) between the incident infrared flux and the output from the infrared sensor. Periodic measurements to reestablish and verify system responsivity are necessary in many applications because the characteristics of the sensors, such as optical efficiency, detector response, optical alignment, offsets and the like vary with time. The calibration process is best accomplished by stimulating the infrared sensor with a known, stable radiometric source which emulates the spatial, spectral, and intensity characteristics of an actual target to the maximum practical extent.
Most infrared spatial scanning and imaging systems use an internal radiometric source for calibration. These calibration sources typically take the forms of heated plates, wires, or resistive elements as, for example, shown in U.S. Pat. Nos. 3,694,624, 4,433,924 and 4,378,489.
These prior art systems have certain limitations. For example, in instances where an AC coupled operation is necessary, these prior art internal calibration sources typically employ a signal chopper, either mechanical or electrical, to produce a fluctuating calibration source within the temporal band pass of the system. Then too, as will be appreciated by those skilled in the art, most of the techniques heretofore used for internal calibration require a close control of the temperature of the calibrating source and the system optics. The emittance of a hot wire typically changes with time and is therefore not suitable for use where long term IR sensor repeatability is a factor.